DESCRIPTION: The precedence effect is a phenomenon that enables us to localize sound in a reverberant environment. When sound is produced in enclosed spaces, such as rooms, the original sound is followed by reflected sound off any hard surface. These reflected sounds, or echoes, are not perceived by the listener as independent sound sources, but rather serve to enhance the original sound's loudness and spatial extent. Traditionally the precedence effect has been regarded as a low level mechanism that inhibits later-arriving sounds. A more cognitive approach will be taken in the proposed experiments. The guiding hypothesis is that echoes carry valuable information about room acoustics. Although the echoes are not heard as separate sounds, the room acoustic information is encoded and analyzed by the brain. Specifically, our hypothesis states that during an ongoing sound, properties of the echo that convey room acoustic information should not change abruptly. Sudden changes of this sort cause a drop in echo threshold. The suppression of the echo is released and the listener hears the echo as a separate, localizable sound. One set of experiments will investigate conditions under which the echo threshold is hypothesized to drop and conditions where it should remain stable. A second set of experiments will investigate listeners' sensitivity to below-threshold changes in the echo's intensity and delay relative to the original sound. The methodology used involves having normal-hearing adults sit in an anechoic chamber, facing an array of loudspeakers that feature an initial or leading sound followed by sounds simulating a variety of echoes. Listeners are asked to press a button when they hear an echo from a particular location (subjective task), or to discriminate a left/right shift in location of the echo (objective task). A final set of experiments will investigate the role of the precedence effect in speech recognition in noisy backgrounds. Previous research has shown that a spatial separation of signal from noise improves speech recognition. The proposed experiments will determine if the spatial separation of perceived auditory images of signal and noise serves the same advantage as their physical separation. Since one of the main problems of the hearing-impaired is separating signal information from a noisy background, the proposed research is relevant to finding solutions to this problem. A better understanding of how the precedence effect aids sound localization in reverberant spaces and its possible role in separating signal from noise should help to devise better hearing aids and design acoustic environments that enhance hearing in general.